Ligand-activated receptors regulate numerous genes, and mediate effects of a broad set of endogenous and exogenous chemicals in vertebrates. Understanding the roles of these transcription factors in zebrafish (Danio rerio) is important to the use of this non-mammalian model in toxicological, pharmacological, and carcinogenesis research. Response to a potential agonist for the pregnane X receptor (Pxr) [pregnenolone (PN)] was examined in developing zebrafish, to assess involvement of Pxr in regulation of selected genes, including genes in cytochrome P450 subfamilies CYP2 and CYP3. We also examined interaction of Pxr and the aryl hydrocarbon receptor (Ahr) signaling pathways. Pregnenolone caused a dose-dependent increase in mRNA levels of pxr, ahr2, CYP1A, CYP2AA1, CYP2AA12, CYP3A65, and CYP3C1, most of which peaked at 3 µM PN. The well-known Ahr agonist 3,3',4,4',5-pentachlorobiphenyl (PCB126) also upregulated expression of pxr, ahr2, CYP1A, CYP2AA12, CYP3A65, and CYP3C1 in a dose-dependent manner. Inhibition of pxr translation by morpholino antisense oligonucleotides (MO) suppressed PN-induced expression of pxr, ahr2, CYP3A65, and CYP3C1 genes. Levels of CYP2AA1 and CYP2AA12 mRNA were increased in the control-MO group exposed to PN; this was prevented by knocking down Pxr. Similarly, Ahr2-MO treatment blocked PCB126-induced mRNA expression of pxr, CYP1A, CYP2AA12, CYP3A65, and CYP3C1. The present study shows self-regulation of pxr by PN in developing zebrafish. Selected zebrafish CYP1, CYP2 (including several CYP2AAs) and CYP3 genes appear to be under the regulation of both Pxr and Ahr2.
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